This invention relates to pulping apparatus for liquid slurry stocks such as paper making stock, and more particularly to rotors for use in such pulping apparatus.
The invention has special relation to pulping apparatus of the type wherein the stock to be pulped is contained in a tub which is provided with a rotor or impeller mounted in the bottom or side wall thereof for rotation on an axis causing outward circulatory movement of the stock in a generally vortical pattern which creates hydraulic shearing forces in the stock and thereby effects the desired pulping or defibering action. Pulping apparatus of this general type is employed in both batch and continuous operations, and the invention is equally applicable to both types of operation.
In the experience of the present inventor, the rotors having the most effective defibering action for the uses outlined above were for a period of about 15 years constructed in accordance with Vokes U.S. Pat. No. 3,073,535. As pointed out in that patent, however, the pumping action of the rotor is provided only by the leading edge surfaces of its vanes and is therefore maintained in close proximity to the cooperating bedplate. As a result, the defibering action, which is caused both by hydraulic shear in the zone above the rotor and by mechanical action between the rotor and bedplate, is extremely effective, but only a relatively small volume of stock is subjected to this action during any given time interval, and the power requirements for adequate treatment of a given batch or for a given period of continuous operation are relatively high.
Another result of the limited pumping action of the rotors of the Vokes patent construction is that because it is primarily confined to the vicinity of the bedplate, it may not develop a complete vortex in the stock above the rotor, especially if the liquid level is relatively high. This in turn can lead to a tendency for floating material to remain on or near the surface of the stock without reaching the vicinity of the rotor for subjection to its defibering action.
The rotors disclosed in my U.S. Pat. No. 3,889,885 were developed to offer improvement over those of the Vokes patent in two respects -- the thickness of the defibering vanes of the rotor was reduced to minimize their pumping action, and pumping vanes were provided on the upper surfaces of certain of the defibering vanes to increase the vortical circulation effect created by the rotor in operation. It has been established in many installations that a rotor constructed in accordance with that patent requires substantially less horsepower per ton, for either batch or continuous operation, than a rotor of the same diameter constructed in accordance with the Vokes patent, to produce substantially the same effective defibering action on any of a wide variety of furnishes, ranging from clean broke or pulp lap to waste paper of low and dirty grades, and at any handleable consistency.